I don't think the debt metaphor works, because there's no real or implied creditor. (Except maybe time or entropy?)

Technical mass might be better. The code keeps accreting, developer efforts orbit around it in ever-decreasing circles, and at some point it becomes impossible to escape from it.

But is this really a technical problem, or a documentation and specification problem? If there's no separate formal model for the project, the code is the documentation, and that's never a good place to be.